1. Field of the Invention
The present invention relates to a method for forming pattern.
2. Discussion of the Background
In the field of microfabrication represented by the manufacture of integrated circuit devices, lithographic technology enabling microfabrication with a line width of 0.10 μm or less has been demanded in recent years in order to increase the degree of integration. However, microfabrication in a subquarter micron level is said to be very difficult using near ultraviolet rays such as i-lines which are generally used as radiation in a common lithography process. Therefore, in order to perform microfabrication with a line width of 0.10 μm or less, use of radiation with a shorter wavelength has been studied. As examples of such short wavelength radiation, a bright line spectrum of a mercury lamp, deep ultraviolet rays represented by excimer lasers, X rays, electron beams, and the like can be given. A KrF excimer laser (wavelength: 248 nm) or an ArF excimer laser (wavelength: 193 nm) are being given particular attention.
As a resist suitable for being irradiated with such an excimer laser, many chemically amplified resists utilizing the chemical amplification effect of a component having an acid-dissociable functional group and a component (hereinafter referred to as “an acid generator”) which generates an acid upon irradiation (hereinafter referred to as “exposure”) have been proposed. As a chemically amplified resist, a resist containing a resin having a t-butyl ester group of carboxylic acid or a t-butyl carbonate group of phenol and an acid generator has been proposed. The t-butyl ester group or t-butyl carbonate group in the resin dissociates by the action of an acid generated upon exposure, whereby the resist is provided with an acidic group such as a carboxylic group or a phenolic hydroxyl group. As a result, the exposed areas of the resist film become readily soluble in an alkaline developer.
Capability of forming more minute patterns (a minute resist pattern with a line width of about 45 nm, for example) will be required for the lithographic process in the future. Reducing the wavelength of a light source of a photolithography instrument and increasing the numerical aperture (NA) of a lens could be a means for forming such a pattern with a line width of less than 45 nm. However, an expensive exposure machine is necessary for reducing the wavelength of a light source. In addition, due to a trade-off relationship between the resolution and the depth of focus, increasing the numerical aperture (NA) of a lens involves a problem of decreasing the depth of focus even if the resolution is increased.
Recently, a liquid immersion lithographic method has been reported as a lithographic technique enabling solution of such a problem. In the process, a layer of a liquid high refractive index medium (liquid for liquid immersion lithography) such as pure water or a fluorine-containing inert liquid is caused to be present during exposure between the lens and the resist film on a substrate, at least on the surface of the resist film. According to this method, an inert gas atmosphere in the light-path space, such as air and nitrogen, is replaced by a liquid with a high refractive index (n), for example, pure water, whereby resolution can be increased without a decrease in the focal depth by using a light source with a given wavelength to the same degree as in the case in which a light source with a shorter wavelength is used or the case in which a higher NA lens is used. Since a resist pattern with a higher resolution excelling in focal depth can be formed at a low cost using a lens mounted in existing apparatuses by using the liquid immersion lithographic method, the method has received a great deal of attention and is currently being put into practice.
Although downsizing of the line width of the above exposure technology is said to be up to 45 nmhp at most, the technological development is advancing toward a 32 nmhp generation requiring further minute fabrication. More recently, in an effort to respond to such device complication and high density requirement a double patterning or double exposure technology of patterning 32 nm LS by producing a rough line pattern or an isolated trench pattern and superposing these patterns displacing one pattern from the other by half a pitch has been proposed in SPIE 2006 61531K and Third International Symposium Immersion Lithography PO-11.
In one proposed example, after forming a 1:3 pitch 32 nm line and after processing a hard mask made from a material such as SiO2 by etching, another 1:3 pitch 32 nm line is formed in a position displaced from the first position by half a pitch, followed by HM processing again by etching. As a result, a 1:1 pitch 32 nm line can be ultimately formed (See Third International Symposium Immersion Lithography PO-11).